Soul degrade. A fatal disease few monsters have suffered from. Those who were born with it, die around the age of 5. Gaster never used the determination experiments to torture Sans.. he did it in an attempt to save him.
New parts come out slowly
(This is my version of HorrorTale)
Frisk seems to abandon the underground. The monster try to move on but the thought of her being gone isn't a nice one. Years later a human child falls into the hell, Frisk had abandoned.